About predicting adult height
How tall will a child grow? The most widely used quick estimate is the mid-parental height method, which predicts a child's adult height from the heights of their two parents. This tool applies that method: enter the mother's height, the father's height, and the child's sex, and it returns a predicted adult height in centimetres and feet-and-inches, along with a typical range around it.
The reasoning is genetic. Height is strongly heritable, roughly 80 percent determined by the genes a child inherits and about 20 percent by environment such as nutrition and health. Since a child gets genes from both parents, the average of the parents' heights is a sensible starting point. The method then adjusts for sex, because adult men are on average taller than adult women, adding 6.5 centimetres for a boy or subtracting 6.5 centimetres for a girl. It is the same back-of-the-envelope calculation pediatricians use during growth checks.
Parents use it out of curiosity, to see whether a child is tracking near their genetic potential, and to set rough expectations. It is an estimate, accurate to within about 5 centimetres for most children, not a precise forecast.
How the formula works
The mid-parental height formula averages the parents and shifts by sex:
Mid-parental height = (Mother's height + Father's height) / 2 Boy = Mid-parental height + 6.5 cm Girl = Mid-parental height - 6.5 cm Typical range = predicted height plus or minus 5 cm
- Average first: add both parents' heights and halve to get the mid-parental value.
- Adjust for sex: the 6.5 cm shift is half the average adult male-female height gap of about 13 cm.
- Add a range: the true result usually falls within 5 cm of the prediction in either direction.
Worked example
A boy with a mother of 165 cm and a father of 178 cm.
- Add the parents: 165 + 178 = 343 cm.
- Average them: 343 / 2 = 171.5 cm (the mid-parental height).
- Adjust for a boy: 171.5 + 6.5 = 178 cm.
- Convert: 178 cm is about 5 feet 10 inches.
- Range: roughly 173 to 183 cm, the typical plus-or-minus 5 cm window.
Sex adjustment and conversions
The 6.5 cm adjustment and some common height conversions for context.
| Item | Value | Note |
|---|---|---|
| Boy adjustment | +6.5 cm | Added to the mid-parental height |
| Girl adjustment | -6.5 cm | Subtracted from the mid-parental height |
| Typical accuracy | ±5 cm | Range around the prediction |
| 1 inch | 2.54 cm | For feet-and-inches conversion |
| Genetics vs environment | ~80% / 20% | Why parents' heights dominate |
Common pitfalls
- Treating the number as exact. It is an estimate with a plus-or-minus 5 cm range; a child can land outside it, especially with late growth spurts.
- Mixing units. Enter both parents in the same unit. Combining feet-inches and centimetres gives a meaningless average.
- Ignoring environment. Poor nutrition, chronic illness, or sleep deprivation can keep a child below their genetic potential.
- Applying it to medical concerns. If a child is far off their growth curve, the formula is no substitute for a pediatrician and a bone-age assessment.
- Forgetting the secular trend. Each generation has tended to grow taller, so children sometimes exceed both parents' heights.
Frequently asked questions
How does the mid-parental height method work?
The mid-parental method estimates a child's adult height from the average of the two parents' heights, adjusted for the child's sex. You add the mother's and father's heights, divide by two to get the mid-parental value, then add 6.5 centimetres (about 2.5 inches) for a boy or subtract 6.5 centimetres for a girl. This adjustment reflects the average height difference between adult men and women. It is the standard quick method pediatricians use and is accurate to within roughly 5 centimetres for most children.
How accurate is a height prediction from parents' heights?
The mid-parental method is an estimate, not a guarantee. It typically predicts adult height to within about 5 centimetres (2 inches) in either direction for most children, but individual results can vary more. Height is roughly 80 percent determined by genetics and 20 percent by environment, including nutrition, sleep, general health, and childhood illnesses. Because many genes contribute and the environment plays a real role, treat the number as a central estimate with a range around it.
Why do boys and girls use a different adjustment?
On average adult men are taller than adult women by roughly 13 centimetres. The mid-parental formula splits that difference: it adds half of it (about 6.5 centimetres) for a boy and subtracts half for a girl, around the average of the parents' heights. This centres the prediction on where a child of that sex is most likely to land relative to their parents. Without the adjustment, the formula would systematically under-predict boys and over-predict girls.
What factors besides genetics affect final height?
Nutrition is the biggest environmental factor: adequate protein, calories, and micronutrients during childhood and the teen growth spurt support reaching genetic potential. Sleep matters because growth hormone is released mainly during deep sleep. Chronic illness, untreated hormonal conditions, and severe stress can reduce final height. Each generation has also tended to be taller than the last as living conditions improved, which is why a child sometimes ends up taller than both parents.
At what age does a person stop growing taller?
Most girls reach close to their final height by around age 14 to 15, and most boys by around 16 to 17, though some boys continue growing into their late teens. Growth stops when the growth plates at the ends of the long bones fuse, which happens after the puberty growth spurt. The mid-parental prediction estimates that eventual adult height, so it is most useful as a forward-looking guide for a child who is still growing.